ENC-Based Obstacles

The reactive algorithm was also tested in situations where the ASV’s planned path went through an obstacle denoted in the ENC, specifically the breakwater near the UNH pier and the UNH pier itself, requiring ENC_OA to provide a new, safe path. The results shown in the following tests are from data collected in the field.

The EchoBoat successfully avoided the breakwater using only the reactive method in two different directions: driving into the cove (Figures 3.7 and 3.8) and leaving the cove (Figures 3.9 and 3.10). The first mission started on the far-side of the breakwater with reference to the UNH

Minimum Distance Between EchoBoat

Table 3.2: Field data on the minimum distance from the EchoBoat to the synthetic ellipse

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pier, in which the EchoBoat’s planned mission was to line-follow through the breakwater. (A plan view of the mission is shown in Figure 3.7.)

Due to the angular profile of the breakwater in reference to the planned path, the southern edge of the breakwater came into the search area first. The images in Figure 3.8 show snapshots of the IvP Functions (left), where the colored lines denote the Heading IvP Functions and the stars denote the desired (black) and current (red) heading, and a plan view of the mission, where the ASV’s current position is denoted with a yellow dot (right). The top images of Figure 3.8 clearly show that headings that lead toward the breakwater (southwest) are penalized by ENC_OA (orange IvP Function in the left image) and the desired heading shifts approximately 30 degrees to the northwest from the planned path. As the EchoBoat navigates around the breakwater, the penalized headings shift from the southwest to the southeast and once the ASV navigates around the

Figure 3.7: Plan view of a mission driving around the breakwater near the UNH Pier using ENC_OA to avoid the breakwater. In this mission, the ASV drove from the far-side of the breakwater (with reference to the UNH pier), around the breakwater, to inside the pier’s cove

on the path shown in blue.

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dangerous area, the EchoBoat begins prioritizing track-following (bottom panels of Figure 3.8).

During this mission, the EchoBoat drove no closer than 3.0 m away from the buffered breakwater.

In the next mission, the ASV avoided the breakwater after starting on the near-side with reference to the UNH pier (a plan-view of the mission is shown in Figure 3.9). During this mission, the ASV avoided the breakwater towards the south instead of the north (unlike the previous

Figure 3.8: Snapshots at the two critical junctions of the mission where the EchoBoat drove around the breakwater while line-following where the left images show the heading based IvP functions for ENC_OA (orange line), the waypoint behavior (blue line) and the combined

IvP Function (green line) as well as the current and desired heading (red and black stars respectively) and a plan view of the mission (right images) where the ASV’s position is

marked with a yellow dot at the time of the IvP Functions.

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mission) as only the northwest corner of the breakwater was visible to ENC_Contact as the breakwater entered into the ASV’s search area. The left panel of Figure 3.10 shows that the penalty of driving around the breakwater to the north, due to ENC_OA (orange line) changing the combined IvP function (green line), was enough to force the EchoBoat to attempt to circumnavigate the breakwater to the south towards Fort Constitution. Once the EchoBoat drove to the southeast corner of the breakwater, the ASV was forced to split the gap between the breakwater and the shallow water polygon (bottom panels of Figure 3.10). As a result, it drove very close to both the breakwater (1.1 m at its closest point) and the shallow water (3.6 m at the closest point). Although the ASV drove very close to the buffered breakwater and shallow water, it successfully avoided all obstacles in its environment and safely arrived at its desired waypoint without human intervention. In these two examples, the ASV safely completed its mission by using

Figure 3.9: Plan view of a mission driving around the breakwater near the UNH Pier in the field using ENC_OA to avoid the breakwater

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ENC_OA to adjust its path around unsafe mission plans in real-time using both waypoint navigation and line following.

In the next mission, the EchoBoat’s planned path drove straight through the pier as shown in Figure 3.11. Similar to previous missions, the ASV stayed on the desired path while it was safe, but when it was hazardous, the ASV left the planned path and avoided the pier. A snapshot of the mission as the ASV approaches the pier is shown in Figure 3.12 where the IvP functions illustrate

Figure 3.10: Snapshots at the two critical junctions of the mission where the EchoBoat drove around the breakwater where the left images show the heading based IvP functions for ENC_OA (orange line), the waypoint behavior (blue line) and the combined IvP Function (green line) as well as the current and desired heading (red and black stars respectively) and a

plan view of the mission (right images) where the ASV’s position is marked with a yellow dot at the time of the IvP Functions.

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that the headings leading toward the UNH pier are penalized. As shown previously, once the EchoBoat safely navigated around the pier, the ASV returned to its desired trackline. Even though

Figure 3.11: Plan view of a mission driving around the UNH Pier in the field using ENC_OA.

The planned path is given with the green, dashed line and the path driven by the EchoBoat is shown in blue.

Figure 3.12: Snapshot during the mission line-following while avoiding the UNH pier while in the field when ENC_OA starts to push the desired heading of the EchoBoat to the southeast to avoid the pier. In the left image, all IvP functions are shown with ENC_OA in

orange, the waypoint behavior in blue, and combined IvP function in green as well as the current and desired heading with a red and black star respectfully.

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the pier is a non-uniform obstacle, ENC_OA allowed for the ASV to safely reach its desired location, while driving no closer than 3.4 m away from the pier, despite poor mission planning.

In some situations, the reactive mission planner can get stuck in local minima. An example mission is shown in Figure 3.13, where the EchoBoat attempted to navigate from one side of the pier to the other. In this mission, the EchoBoat oscillates between heading choices to achieve the waypoint and those that are penalized by the presence of the pier. This cycle was repeated ad infinitum as the EchoBoat was stuck in a local minimum and the mission had to be canceled.

Snapshots of the heading IvP Functions, shown in Figure 3.14, illustrate the critical junctions of the ASV path as it got stuck in a local minimum. Although this behavior is not ideal, the ASV came no closer than 8.3 m away from the pier. This result illustrates that reactive behaviors alone may be insufficient for reliable navigation and that additional behaviors that look with a greater

Figure 3.13: Plan view of a mission driving towards the UNH Pier in the field where the EchoBoat got stuck in a local minimum. The path driven by the EchoBoat is shown in blue

and the desired waypoint is shown with a pink star.

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field of interest, such as running Depth-Based A* when stuck in a local minimum, is required.

Implementation of these behaviors are left for future work.

Figure 3.14: Snapshots at the three critical junctions of the mission where the EchoBoat got stuck in a local minimum. The left images show the heading-based IvP functions for ENC_OA (orange line), the waypoint behavior (blue line) and the combined IvP Function (green line) as well as the current and desired heading (red and black stars respectively) and a

plan view of the mission (right images) where the ASV’s position is marked with a yellow star at the time of the IvP Functions.

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These manufactured scenarios of purposely driving at the pier, breakwater and synthetic ellipse show that the reactive obstacle avoidance procedures keep the ASV from hitting charted obstacles. Although charted obstacles can typically be avoided through mission planners (like the Depth-Based A* mission planner), they must still be accounted for in real time as other behaviors (i.e., avoiding uncharted obstacles or vessels) might cause the ASV to deviate from the planned path. Using the reactive obstacle avoidance system developed in this work, charted obstacles are accounted for and are avoided in real time without suffering from the challenges of real-time sensor processing. However, to further ensure the ASV’s safety, especially around uncharted obstacles, additional real-time obstacle avoidance systems should be implemented that use external sensors.

The implementation of these systems has been left for future work.